Generally, a conventional diesel engine has one needle valve and one spring, wherein the needle valve is opened when fuel having pressure higher than opening pressure is introduced and is closed when the fuel having lower pressure is introduced. In this manner, if the pressure higher than the opening pressure is formed within a fuel valve when fuel of high pressure formed in a fuel pump enters to the fuel valve, the spring lifts the needle valve against a force pressing against the needle valve through pressure of fuel oil, such that the fuel is injected into a cylinder through multiple nozzle holes located on an end terminal of a nozzle. Such a method consists of one mechanism in which all nozzle holes are opened according to a predefined opening pressure, and thus, after the opening pressure is formed, even if higher pressure is introduced to the fuel valve, injection is only continued through a predefined number of nozzle holes. In case of a gas engine, two fuel injection valves, one for a pilot valve and one for a valve using primarily gas, or two needle cylinders are provided to be used, and for two fuel supply paths corresponding thereto, a pipe line that conforms to two valves needs to be installed, thereby requiring two governors for timing control, a linkage device, etc.
In case of a Wartsila-Sulzer approach, an MAN-B & W approach, and a medium-size engine, which are a representative form of a conventional fuel valve, in the Wartsila-Sulzer approach, when pressure is greater than the opening pressure but fails to form a high pressure, fuel flows into the cylinder rather than being injected into the cylinder through multiple nozzle holes fabricated on the nozzle. Also, even after the fuel injection is finished, a space (SAC volume) between the closed needle valve and the nozzle hole is large such that residue fuel remaining in this space flows into the cylinder, thereby causing the problem as described above.
Also, in the MAN-B & W approach, a needle valve in a slide type is adopted to reduce the SAC volume, however, the MAN-B & W approach has limitation in that pressure above the opening pressure cannot be actively coped with. In other words, although the SAC volume is fixed, the SAC volume, which is a space between a low-pressure needle valve and the nozzle, can be minimized after the injection is finished. The fuel injection valve in a dual fuel form is operated with two cylinders, thereby requiring two governors and two links.